Golf ball

ABSTRACT

A golf ball  2  includes a spherical core  4 , a mid layer  6  outside the core  4 , and a cover  8  outside the mid layer  6 . The core  4  includes a spherical center  10  and an envelope layer  12  outside the center  10 . At all points P included in a zone that extends over a distance range from equal to or greater than 1 mm to equal to or less than 15 mm from the central point of the core  4 , the following mathematical formula is satisfied.
 
−5&lt; H 2− H 1&lt;5
 
     In the mathematical formula, H 1  indicates the JIS-C hardness at a point P 1  located radially inward of each point P at a distance of 1 mm from the point P, and H 2  indicates the JIS-C hardness at a point P 2  located radially outward of each point P at a distance of 1 mm from the point P.

This application claims priority on Patent Application No. 2010-227077 filed in JAPAN on Oct. 7, 2010. The entire contents of this Japanese Patent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to golf balls. Specifically, the present invention relates to multi-piece golf balls that include a center, an envelope layer, a mid layer, and a cover.

2. Description of the Related Art

Golf players' foremost requirement for golf balls is flight performance. Golf players place importance on flight performance upon shots with a driver, a long iron, and a middle iron. Flight performance correlates with the resilience performance of a golf ball. When a golf ball having excellent resilience performance is hit, the golf ball flies at a high speed, thereby achieving a large flight distance.

An appropriate trajectory height is required in order to achieve a large flight distance. A trajectory height depends on a spin rate and a launch angle. In a golf ball that achieves a high trajectory by a high spin rate, a flight distance is insufficient. In a golf ball that achieves a high trajectory by a high launch angle, a large flight distance is obtained. Use of a core having an outer-hard/inner-soft structure can achieve a low spin rate and a high launch angle.

Golf players also place importance on spin performance of golf balls. When a backspin rate is high, the run is short. It is easy for golf players to cause a golf ball, to which backspin is easily provided, to stop at a target point. When a sidespin rate is high, the golf ball tends to curve. It is easy for golf players to intentionally cause a golf ball, to which sidespin is easily provided, to curve. A golf ball to which spin is easily provided has excellent controllability. In particular, advanced golf players place importance on controllability upon a shot with a short iron.

In light of achieving various performance characteristics, golf balls each having a multilayer structure have been proposed. JP10-328326 (U.S. Pat. No. 6,468,169) discloses a golf ball that includes a core, an envelope layer, an inner cover, and an outer cover. JP2001-17575 (U.S. Pat. No. 6,271,296) discloses a golf ball that includes a core, an envelope layer, a mid layer, and a cover. JP2002-272880 (U.S. Pat. No. 6,913,547) discloses a golf ball that includes a core and a cover. The core consists of a center and an outer core layer. The cover consists of an inner cover layer and an outer cover layer. JP2003-205052 (US 2003/0166422) discloses a golf ball that includes a center, a mid layer, and a cover. JP2004-130072 (US2004/0029648) discloses a golf ball that includes a core and a cover. The core has a three-layer structure.

When a core that has an outer-hard/inner-soft structure and an excessive hardness distribution is hit with a driver, the energy loss is high in the core. The energy loss impairs resilience performance. When a core that has an outer-hard/inner-soft structure and an excessive hardness distribution is hit with a short iron, the spin rate is low. The low spin rate impairs controllability.

An object of the present invention is to provide a golf ball that provides a large flight distance when being hit with a driver and that has excellent controllability when being hit with a short iron.

SUMMARY OF THE INVENTION

A golf ball according to the present invention comprises a core, a mid layer positioned outside the core, and a cover positioned outside the mid layer. The core comprises a center and an envelope layer positioned outside the center. A ratio of a volume of the core to a volume of a phantom sphere of the golf ball is equal to or greater than 76%. A difference (He−Ho) between a JIS-C hardness He at a surface of the core and a JIS-C hardness Ho at a central point of the core is equal to or greater than 15 but equal to or less than 30. A JIS-C hardness Hc of the cover is less than the hardness Ho. At all points P included in a zone that extends over a distance range from equal to or greater than 1 mm to equal to or less than 15 mm from the central point of the core, the following mathematical formula is satisfied. −5<H2−H1<5 In the mathematical formula, H1 indicates a JIS-C hardness at a point P1 that is located radially inward of each point P at a distance of 1 mm from the point P, and H2 indicates a JIS-C hardness at a point P2 that is located radially outward of each point P at a distance of 1 mm from the point P.

In the golf ball according to the present invention, a hardness distribution of the core is appropriate. The core has a low energy loss when being hit with a driver. When the golf ball is hit with a driver, a large flight distance is obtained. The golf ball has excellent controllability when being hit with a short iron.

Preferably, the JIS-C hardness Hc of the cover is equal to or less than 65. Preferably, a thickness of the cover is equal to or less than 0.8 mm.

Preferably, a JIS-C hardness Hm of the mid layer is equal to or greater than 90. Preferably, a thickness of the mid layer is equal to or less than 1.5 mm.

The cover can be formed from a resin composition. Preferably, a principal component of a base material of the resin composition is a thermoplastic polyurethane. Preferably, a shear loss elastic modulus G″ of the resin composition, which is measured under conditions of a vibration frequency of 10 Hz and a temperature of 0° C., is equal to or less than 1.95×10⁷ Pa, and a ratio (E″/G″) of a tensile loss elastic modulus E″ of the resin composition, which is measured under the same conditions, to the shear loss elastic modulus G″ is equal to or greater than 1.76. Preferably, a polyol component of the thermoplastic polyurethane is polytetramethylene ether glycol having a number average molecular weight of 1500 or less.

Preferably, a diameter of the center is equal to or greater than 10 mm but equal to or less than 20 mm. Preferably, a thickness of the envelope layer is equal to or greater than 8 mm but equal to or less than 18 mm. Preferably, a thickness of the mid layer is equal to or less than 1.2 mm. Preferably, a thickness of the cover is equal to or less than 0.5 mm.

Preferably, a difference (Ho−Hc) between the hardness Ho and the hardness Hc is equal to or greater than 15 but equal to or less than 40.

The center can be formed by a rubber composition being crosslinked. The envelope layer can be formed by another rubber composition being crosslinked. Preferably, the rubber composition of the center does not include an organic sulfur compound, and the rubber composition of the envelope layer includes an organic sulfur compound.

Preferably, the golf ball further comprises an adhesive layer between the mid layer and the cover. The adhesive layer is formed from an adhesive. A base polymer of the adhesive is a two-component curing type epoxy resin obtained by curing a bisphenol A type epoxy resin with a curing agent including a polyamine compound. A gel fraction of the adhesive is equal to or greater than 40% but equal to or less than 80%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway cross-sectional view of a golf ball according to one embodiment of the present invention;

FIG. 2 is a graph showing a hardness distribution of a core of a golf ball according to Example 1 of the present invention;

FIG. 3 is a graph showing a hardness distribution of a core of a golf ball according to Example 2 of the present invention;

FIG. 4 is a graph showing a hardness distribution of each core of golf balls according to Examples 3 to 7 of the present invention and Comparative Example 4;

FIG. 5 is a graph showing a hardness distribution of a core of a golf ball according to Example 8 of the present invention;

FIG. 6 is a graph showing a hardness distribution of a core of a golf ball according to Comparative Example 1;

FIG. 7 is a graph showing a hardness distribution of a core of a golf ball according to Comparative Example 2; and

FIG. 8 is a graph showing a hardness distribution of a core of a golf ball according to Comparative Example 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention based on preferred embodiments with reference to the accompanying drawings.

A golf ball 2 shown in FIG. 1 includes a spherical core 4, a mid layer 6 positioned outside the core 4, and a cover 8 positioned outside the mid layer 6. The core 4 includes a spherical center 10 and an envelope layer 12 positioned outside the center 10. On the surface of the cover 8, a large number of dimples 14 are formed. Of the surface of the golf ball 2, a part other than the dimples 14 is a land 16. The golf ball 2 includes a paint layer and a mark layer on the external side of the cover 8 although these layers are not shown in the drawing.

The golf ball 2 has a diameter of 40 mm or greater but 45 mm or less. From the standpoint of conformity to the rules established by the United States Golf Association (USGA), the diameter is preferably equal to or greater than 42.67 mm. In light of suppression of air resistance, the diameter is preferably equal to or less than 44 mm and more preferably equal to or less than 42.80 mm. The golf ball 2 has a weight of 40 g or greater but 50 g or less. In light of attainment of great inertia, the weight is preferably equal to or greater than 44 g and more preferably equal to or greater than 45.00 g. From the standpoint of conformity to the rules established by the USGA, the weight is preferably equal to or less than 45.93 g.

Preferably, the center 10 is obtained by crosslinking a rubber composition. Examples of preferable base rubbers for use in the rubber composition include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers, and natural rubbers. In light of resilience performance, polybutadienes are preferred. When a polybutadiene and another rubber are used in combination, it is preferred if the polybutadiene is included as a principal component. Specifically, the proportion of the polybutadiene to the entire base rubber is preferably equal to or greater than 50% by weight and more preferably equal to or greater than 80% by weight. The proportion of cis-1,4 bonds in the polybutadiene is preferably equal to or greater than 40% and more preferably equal to or greater than 80%.

The rubber composition of the center 10 includes a co-crosslinking agent. The co-crosslinking agent achieves high resilience of the center 10. Examples of preferable co-crosslinking agents in light of resilience performance include monovalent or bivalent metal salts of an α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. Specific examples of preferable co-crosslinking agents include zinc acrylate, magnesium acrylate, zinc methacrylate, and magnesium methacrylate. In light of releasability of the center from a mold and resilience performance of the golf ball, magnesium acrylate and magnesium methacrylate are preferred.

In light of achievement of both desired releasability of the center from a mold and desired resilience performance of the golf ball 2, the amount of the co-crosslinking agent is preferably equal to or greater than 20 parts by weight but equal to or less than 50 parts by weight, per 100 parts by weight of the base rubber. The amount is particularly preferably equal to or greater than 25 parts by weight per 100 parts by weight of the base rubber. The amount is more preferably equal to or less than 45 parts by weight and particularly preferably equal to or less than 40 parts by weight, per 100 parts by weight of the base rubber.

An α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms and a metal oxide may be included in the rubber composition. They both react with each other in the rubber composition to obtain a salt. The salt serves as a co-crosslinking agent. Examples of preferable α,β-unsaturated carboxylic acids include acrylic acid and methacrylic acid. Examples of preferable metal oxides include zinc oxide and magnesium oxide. In light of releasability, magnesium oxide is particularly preferred. The amount of the α,β-unsaturated carboxylic acid is preferably equal to or greater than 15 parts by weight but equal to or less than 45 parts by weight, per 100 parts by weight of the base rubber. The amount of the α,β-unsaturated carboxylic acid is particularly preferably equal to or greater than 20 parts by weight per 100 parts by weight of the base rubber. The amount of the α,β-unsaturated carboxylic acid is particularly preferably equal to or less than 40 parts by weight per 100 parts by weight of the base rubber. The amount of the metal oxide is preferably equal to or greater than 20 parts by weight but equal to or less than 50 parts by weight, per 100 parts by weight of the base rubber. The amount of the metal oxide is particularly preferably equal to or greater than 25 parts by weight per 100 parts by weight of the base rubber. The amount of the metal oxide is particularly preferably equal to or less than 45 parts by weight per 100 parts by weight of the base rubber.

Preferably, the rubber composition of the center 10 includes an organic peroxide together with a co-crosslinking agent. The organic peroxide serves as a crosslinking initiator. The organic peroxide contributes to the resilience performance of the golf ball 2. Examples of suitable organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. In light of versatility, dicumyl peroxide is preferred.

In light of resilience performance of the golf ball 2, the amount of the organic peroxide is preferably equal to or greater than 0.1 parts by weight, more preferably equal to or greater than 0.2 parts by weight, and particularly preferably equal to or greater than 0.3 parts by weight, per 100 parts by weight of the base rubber. In light of soft feel at impact, the amount of the organic peroxide is preferably equal to or less than 1.5 parts by weight, more preferably equal to or less than 1.0 parts by weight, and particularly preferably equal to or less than 0.8 parts by weight, per 100 parts by weight of the base rubber.

The rubber composition of the center 10 may include an organic sulfur compound. Examples of preferable organic sulfur compounds include monosubstitutions such as diphenyl disulfide, bis(4-chlorophenyl)disulfide, bis(3-chlorophenyl)disulfide, bis(4-bromophenyl)disulfide, bis(3-bromophenyl)disulfide, bis(4-fluorophenyl)disulfide, bis(4-iodophenyl)disulfide, bis(4-cyanophenyl)disulfide, and the like; disubstitutions such as bis(2,5-dichlorophenyl)disulfide, bis(3,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,5-dibromophenyl)disulfide, bis(3,5-dibromophenyl)disulfide, bis(2-chloro-5-bromophenyl)disulfide, bis(2-cyano-5-bromophenyl)disulfide, and the like; trisubstitutions such as bis(2,4,6-trichlorophenyl)disulfide, bis(2-cyano-4-chloro-6-bromophenyl)disulfide, and the like; tetrasubstitutions such as bis(2,3,5,6-tetrachlorophenyl)disulfide and the like; and pentasubstitutions such as bis(2,3,4,5,6-pentachlorophenyl)disulfide, bis(2,3,4,5,6-pentabromophenyl)disulfide, and the like. The organic sulfur compound contributes to resilience performance. Particularly preferable organic sulfur compounds are diphenyl disulfide and bis(pentabromophenyl)disulfide.

In light of resilience performance of the golf ball 2, the amount of the organic sulfur compound is preferably equal to or greater than 0.1 parts by weight and more preferably equal to or greater than 0.2 parts by weight, per 100 parts by weight of the base rubber. In light of soft feel at impact, the amount of the organic sulfur compound is preferably equal to or less than 1.5 parts by weight, more preferably equal to or less than 1.0 parts by weight, and particularly preferably equal to or less than 0.8 parts by weight, per 100 parts by weight of the base rubber.

For the purpose of adjusting specific gravity and the like, a filler may be included in the center 10. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. The amount of the filler is determined as appropriate so that the intended specific gravity of the center 10 is accomplished. A particularly preferable filler is zinc oxide. Zinc oxide serves not only as a specific gravity adjuster but also as a crosslinking activator.

According to need, an anti-aging agent, a coloring agent, a plasticizer, a dispersant, sulfur, an vulcanization accelerator, and the like are added to the rubber composition of the center 10. Crosslinked rubber powder or synthetic resin powder may also be dispersed in the rubber composition.

In light of resilience performance, the center 10 has a central hardness Ho of preferably 40 or greater, more preferably 45 or greater, and particularly preferably 50 or greater. In light of suppression of spin, the central hardness Ho is preferably equal to or less than 80, more preferably equal to or less than 75, and particularly preferably equal to or less than 70. The central hardness Ho is measured by pressing a JIS-C type hardness scale against the central point of a cut plane of the center 10 that has been cut into two halves. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used.

The hardness of the center 10 gradually increases from its central point toward its surface. The center 10 has a surface hardness greater than the central hardness Ho.

The center 10 has a diameter of preferably 10 mm or greater but 20 mm or less. The center 10 having a diameter of 10 mm or greater can achieve excellent feel at impact. In this respect, the diameter is more preferably equal to or greater than 12 mm and particularly preferably equal to or greater than 13 mm. When the center 10 has a diameter of 20 mm or less, the envelope layer 12 having a sufficiently large thickness can be formed. In this respect, the diameter is more preferably equal to or less than 18 mm and particularly preferably equal to or less than 17 mm.

The envelope layer 12 is obtained by crosslinking a rubber composition. Examples of base rubbers for use in the rubber composition include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers, and natural rubbers. In light of resilience performance, polybutadienes are preferred. When a polybutadiene and another rubber are used in combination, it is preferred if the polybutadiene is included as a principal component. Specifically, the proportion of the polybutadiene to the entire base rubber is preferably equal to or greater than 50% by weight and more preferably equal to or greater than 80% by weight. The proportion of cis-1,4 bonds in the polybutadiene is preferably equal to or greater than 40% and more preferably equal to or greater than 80%.

In order to crosslink the envelope layer 12, a co-crosslinking agent is preferably used. Examples of preferable co-crosslinking agents in light of resilience performance include monovalent or bivalent metal salts of an α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. Specific examples of preferable co-crosslinking agents include zinc acrylate, magnesium acrylate, zinc methacrylate, and magnesium methacrylate. In light of resilience performance, zinc acrylate and zinc methacrylate are particularly preferred.

In light of resilience performance of the golf ball 2, the amount of the co-crosslinking agent is preferably equal to or greater than 20 parts by weight and particularly preferably equal to or greater than 25 parts by weight, per 100 parts by weight of the base rubber. In light of soft feel at impact, the amount of the co-crosslinking agent is preferably equal to or less than 60 parts by weight, more preferably equal to or less than 50 parts by weight, and particularly preferably equal to or less than 45 parts by weight, per 100 parts by weight of the base rubber.

Preferably, the rubber composition of the envelope layer 12 includes an organic peroxide together with a co-crosslinking agent. The organic peroxide serves as a crosslinking initiator. The organic peroxide contributes to the resilience performance of the golf ball 2. Examples of suitable organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. In light of versatility, dicumyl peroxide is preferred.

In light of resilience performance of the golf ball 2, the amount of the organic peroxide is preferably equal to or greater than 0.1 parts by weight, more preferably equal to or greater than 0.3 parts by weight, and particularly preferably equal to or greater than 0.5 parts by weight, per 100 parts by weight of the base rubber. In light of soft feel at impact, the amount of the organic peroxide is preferably equal to or less than 2.0 parts by weight, more preferably equal to or less than 1.5 parts by weight, and particularly preferably equal to or less than 1.0 parts by weight, per 100 parts by weight of the base rubber.

Preferably, the rubber composition of the envelope layer 12 includes an organic sulfur compound. The organic sulfur compounds described above for the center 10 can be used for the envelope layer 12. In light of resilience performance of the golf ball 2, the amount of the organic sulfur compound is preferably equal to or greater than 0.1 parts by weight and more preferably equal to or greater than 0.2 parts by weight, per 100 parts by weight of the base rubber. In light of soft feel at impact, the amount of the organic sulfur compound is preferably equal to or less than 1.5 parts by weight, more preferably equal to or less than 1.0 parts by weight, and particularly preferably equal to or less than 0.8 parts by weight, per 100 parts by weight of the base rubber.

The golf ball 2 may include a core 4 in which the rubber composition of the center 10 does not include an organic sulfur compound and the rubber composition of the envelope layer 12 includes an organic sulfur compound. In the core 4, an appropriate hardness distribution can be obtained.

For the purpose of adjusting specific gravity and the like, a filler may be included in the envelope layer 12. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. Powder of a metal with a high specific gravity may be included as a filler. Specific examples of metals with a high specific gravity include tungsten and molybdenum. The amount of the filler is determined as appropriate so that the intended specific gravity of the envelope layer 12 is accomplished. A particularly preferable filler is zinc oxide. Zinc oxide serves not only as a specific gravity adjuster but also as a crosslinking activator. According to need, various additives such as sulfur, an anti-aging agent, a coloring agent, a plasticizer, a dispersant, and the like are included in the envelope layer 12 in an adequate amount. Crosslinked rubber powder or synthetic resin powder may also be included in the envelope layer 12.

During formation of the envelope layer 12, the center 10 is covered with two uncrosslinked or semi-crosslinked half shells. These half shells are compressed and heated. By this heating, a crosslinking reaction takes place to complete the envelope layer 12. The crosslinking temperature is generally equal to or higher than 140° C. but equal to or lower than 180° C. The time period for crosslinking the envelope layer 12 is generally equal to or longer than 10 minutes but equal to or shorter than 60 minutes.

The hardness of the envelope layer 12 gradually increases from its innermost portion to its surface. In light of resilience performance, a hardness He at the surface of the envelope layer 12 (namely, the surface of the core 4) is preferably equal to or greater than 70 and particularly preferably equal to or greater than 75. In light of feel at impact, the hardness He is preferably equal to or less than 90, more preferably equal to or less than 88, and particularly preferably equal to or less than 87. The hardness He is measured by pressing a JIS-C type hardness scale against the surface of the core 4. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used.

In light of suppression of spin, the difference (He−Hi) between the surface hardness He of the envelope layer 12 and a hardness Hi at the innermost portion of the envelope layer 12 is preferably equal to or greater than 10, more preferably equal to or greater than 12, and particularly preferably equal to or greater than 15. In light of ease of production and durability, the difference (He−Hi) is preferably equal to or less than 25.

The hardness Hi is measured for a hemisphere obtained by cutting the core 4. The hardness Hi is measured by pressing a JIS-C type hardness scale against the cut plane of the hemisphere. The hardness scale is pressed against a region surrounded by a first circle and a second circle. The first circle is the boundary between the center 10 and the envelope layer 12. The second circle is concentric with the first circle and has a radius larger than the radius of the first circle by 1 mm. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used.

The envelope layer 12 has a thickness of preferably 8 mm or greater but 18 mm or less. The envelope layer 12 having a thickness of 8 mm or greater can suppress spin. In this respect, the thickness is more preferably equal to or greater than 9 mm and particularly preferably equal to or greater than 10 mm. When the envelope layer 12 has a thickness of 18 mm or less, the center 10 having a large diameter can be formed. The center 10 having a large diameter can suppress spin. In this respect, the thickness is more preferably equal to or less than 16 mm and particularly preferably equal to or less than 15 mm.

In light of suppression of spin, the difference (He−Ho) between the surface hardness He of the core 4 and the central hardness Ho of the center 10 is preferably equal to or greater than 15 and particularly preferably equal to or greater than 18. In light of ease of production and resilience performance of the core 4, the difference (He−Ho) is preferably equal to or less than 30 and particularly preferably equal to or less than 25.

At all points P included in a zone that extends over a distance range from equal to or greater than 1 mm to equal to or less than 15 mm from the central point of the core 4, the following mathematical formula is satisfied. −5<H2−H1<5

In this mathematical formula, H1 indicates the JIS-C hardness at a point P1. The point P1 is located radially inward of each point P. The distance from the point P to the point P1 is 1 mm. In this mathematical formula, H2 indicates the JIS-C hardness at a point P2. The point P2 is located radially outward of each point P. The distance from the point P to the point P2 is 1 mm. The hardnesses H1 and H2 are measured by pressing a JIS-C type hardness scale against a cut plane of the core 4 that has been cut into two halves. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used. Preferably, at all the points P included in the zone that extends over the distance range from equal to or greater than 1 mm to equal to or less than 15 mm from the central point of the core 4, the following mathematical formula is satisfied. 0<H2−H1<3

The ratio of the volume of the core 4 to the volume of a phantom sphere of the golf ball 2 is equal to or greater than 76%. In other words, the core 4 is large. The core 4 can achieve excellent resilience performance of the golf ball 2. The core 4 can suppress spin of the golf ball 2. In this respect, the ratio is more preferably equal to or greater than 79% and particularly preferably equal to or greater than 80%. The surface of the phantom sphere is the surface of the golf ball 2 when it is postulated that no dimple 14 exists.

A resin composition is suitably used for the mid layer 6. Examples of the base polymer of the resin composition include ionomer resins, styrene block-containing thermoplastic elastomers, thermoplastic polyester elastomers, thermoplastic polyamide elastomers, and thermoplastic polyolefin elastomers.

Particularly preferable base polymers are ionomer resins. Ionomer resins are highly elastic. As described later, the cover 8 of the golf ball 2 is thin and flexible. Thus, when the golf ball 2 is hit with a driver, the mid layer 6 significantly deforms. The mid layer 6 including an ionomer resin contributes to resilience performance upon a shot with a driver. An ionomer resin and another resin may be used in combination. In this case, in light of resilience performance, the proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 70% by weight, and particularly preferably equal to or greater than 85% by weight.

Examples of preferable ionomer resins include binary copolymers formed with an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. A preferable binary copolymer includes 80% by weight or more but 90% by weight or less of an α-olefin, and 10% by weight or more but 20% by weight or less of an α,β-unsaturated carboxylic acid. The binary copolymer has excellent resilience performance. Examples of other preferable ionomer resins include ternary copolymers formed with: an α-olefin; an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and an α,β-unsaturated carboxylate ester having 2 to 22 carbon atoms. A preferable ternary copolymer includes 70% by weight or more but 85% by weight or less of an α-olefin, 5% by weight or more but 30% by weight or less of an α,β-unsaturated carboxylic acid, and 1% by weight or more but 25% by weight or less of an α,β-unsaturated carboxylate ester. The ternary copolymer has excellent resilience performance. For the binary copolymer and the ternary copolymer, preferable α-olefins are ethylene and propylene, while preferable α,β-unsaturated carboxylic acids are acrylic acid and methacrylic acid. A particularly preferable ionomer resin is a copolymer formed with ethylene and acrylic acid or methacrylic acid.

In the binary copolymer and the ternary copolymer, some of the carboxyl groups are neutralized with metal ions. Examples of metal ions for use in neutralization include sodium ion, potassium ion, lithium ion, zinc ion, calcium ion, magnesium ion, aluminum ion, and neodymium ion. The neutralization may be carried out with two or more types of metal ions. Particularly suitable metal ions in light of resilience performance and durability of the golf ball 2 are sodium ion, zinc ion, lithium ion, and magnesium ion.

Specific examples of ionomer resins include trade names “Himilan 1555”, “Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan 1856”, “Himilan 1855”, “Himilan AM7311”, “Himilan AM7315”, “Himilan AM7317”, “Himilan AM7318”, “Himilan AM7329”, “Himilan MK7320”, and “Himilan MK7329”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.; trade names “Surlyn 6120”, “Surlyn 6910”, “Surlyn 7930”, “Surlyn 7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn 8940”, “Surlyn 8945”, “Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”, “Surlyn 9945”, “Surlyn AD8546”, “HPF1000”, and “HPF2000”, manufactured by E.I. du Pont de Nemours and Company; and trade names “IOTEK 7010”, “IOTEK 7030”, “IOTEK 7510”, “IOTEK 7520”, “IOTEK 8000”, and “IOTEK 8030”, manufactured by ExxonMobil Chemical Corporation.

Two or more ionomer resins may be used in combination for the mid layer 6. An ionomer resin neutralized with a monovalent metal ion, and an ionomer resin neutralized with a bivalent metal ion may be used in combination.

The mid layer 6 may include a highly elastic resin. Examples of highly elastic resins include polybutylene terephthalate, polyphenylene ether, polyethylene terephthalate, polysulfone, polyethersulfone, polyphenylene sulfide, polyarylate, polyamide imide, polyetherimide, polyether ether ketone, polyimide, polytetrafluoroethylene, polyamino bismaleimide, polybisamide triazole, polyphenylene oxide, polyacetal, polycarbonate, acrylonitrile-butadiene-styrene copolymers, and acrylonitrile-styrene copolymers.

According to need, a coloring agent such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the mid layer 6 in an adequate amount. For forming the mid layer 6, known methods such as injection molding, compression molding, and the like can be used.

The mid layer 6 has a hardness Hm of preferably 90 or greater. The mid layer 6 having a hardness Hm of 90 or greater achieves excellent resilience performance of the golf ball 2. The mid layer 6 having a hardness Hm of 90 or greater can achieve an outer-hard/inner-soft structure of the sphere consisting of the core 4 and the mid layer 6. The sphere having the outer-hard/inner-soft structure suppress spin of the golf ball 2. In these respects, the hardness Hm is particularly preferably equal to or greater than 92. In light of feel at impact, the hardness Hm is equal to or less than 98 and particularly preferably equal to or less than 97. In light of suppression of spin, preferably, the hardness Hm of the mid layer 6 is greater than the surface hardness He of the core 4, and the surface hardness He of the core 4 is greater than the surface hardness of the center 10.

The hardness Hm is measured with an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.) to which a JIS-C type spring hardness scale is mounted. For the measurement, a slab formed by hot press and having a thickness of about 2 mm is used. A slab kept at 23° C. for two weeks is used for the measurement. At the measurement, three slabs are stacked. A slab formed from a resin composition that is the same as the resin composition of the mid layer 6 is used for the measurement.

In light of suppression of spin, the mid layer 6 has a thickness of preferably 0.3 mm or greater and particularly preferably 0.5 mm or greater. In light of feel at impact, the thickness is preferably equal to or less than 1.5 mm, more preferably equal to or less than 1.2 mm, and particularly preferably equal to or less than 1.0 mm.

The cover 8 is formed from a resin composition. Examples of the base polymer of the resin composition include polyurethanes, polyesters, polyamides, polyolefins, polystyrenes, and ionomer resins. Particularly, polyurethanes are preferred. Polyurethanes are flexible. When the golf ball 2 with the cover 8 including a polyurethane is hit with a short iron, the spin rate is high. The cover 8 formed from a polyurethane contributes to the controllability upon a shot with a short iron. The polyurethane also contributes to the scuff resistance of the cover 8.

When the golf ball 2 is hit with a driver, a long iron, or a middle iron, the sphere consisting of the core 4 and the mid layer 6 becomes significantly distorted since the head speed is high. Since this sphere has an outer-hard/inner-soft structure, the spin rate is suppressed. The suppression of the spin rate achieves a large flight distance. When the golf ball 2 is hit with a short iron, this sphere becomes less distorted since the head speed is low. When the golf ball 2 is hit with a short iron, the behavior of the golf ball 2 mainly depends on the cover 8. Since the cover 8 including the polyurethane is flexible, a high spin rate is obtained. The high spin rate achieves excellent controllability. In the golf ball 2, both desired flight performance upon shots with a driver, a long iron, and a middle iron and desired controllability upon a shot with a short iron are achieved.

When the golf ball 2 is hit, the cover 8 including the polyurethane absorbs the shock. This absorption achieves soft feel at impact. Particularly, when the golf ball 2 is hit with a short iron or a putter, the cover 8 achieves excellent feel at impact.

When being hit, compressive stress is applied to the cover 8 due to movement of the head of a golf club. Since the face of the golf club has a loft angle, shear stress is also applied to the cover 8 when being hit. The head speed of a short iron is low, and the loft angle of a short iron is high. Thus, when the golf ball 2 is hit with a short iron, the shear stress greatly influences the deformation behavior of the cover 8. The head speed of a driver is high, and the loft angle of a driver is low. Thus, when the golf ball 2 is hit with a driver, the compressive stress greatly influences the deformation behavior of the cover 8.

The cover 8 has a shear loss elastic modulus G″ of preferably 1.95×10⁷ Pa or less. As described above, when being hit with a short iron, the deformation behavior of the cover 8 is greatly influenced by the shear stress. The spin rate obtained when being hit with a short iron correlates with the shear loss elastic modulus G″. When the golf ball 2 with the cover 8 having a shear loss elastic modulus G″ of 1.95×10⁷ Pa or less is hit with a short iron, the spin rate is high. The cover 8 can achieve excellent controllability. In this respect, the shear loss elastic modulus G″ is particularly equal to or less than 1.83×10⁷ Pa. In light of ease of forming the cover 8, the shear loss elastic modulus G″ is preferably equal to or greater than 1.00×10⁶ Pa and particularly equal to or greater than 1.10×10⁶ Pa.

The ratio (E″/G″) of a tensile loss elastic modulus E″ of the cover 8 to the shear loss elastic modulus G″ is preferably equal to or greater than 1.76. As described above, when being hit with a driver, the deformation behavior of the cover 8 is greatly influenced by the compressive stress. The spin rate obtained when being hit with a driver correlates with the tensile loss elastic modulus E″. When the golf ball 2 with the cover 8 having a ratio (E″/G″) of 1.76 or greater is hit with a driver, the spin rate is low, and when the golf ball 2 is hit with a short iron, the spin rate is high. In this respect, the ratio (E″/G″) is more preferably equal to or greater than 1.86 and particularly preferably equal to or greater than 1.90. In light of ease of forming the cover 8, the ratio (E″/G″) is preferably equal to or less than 6.0 and particularly preferably equal to or less than 5.5.

The tensile loss elastic modulus E″ is preferably equal to or greater than 2.00×10⁷ Pa, more preferably equal to or greater than 2.20×10⁷ Pa, and particularly preferably equal to or greater than 2.40×10⁷ Pa. The tensile loss elastic modulus E″ is preferably equal to or less than 1.00×10⁸ Pa.

The shear loss elastic modulus G″ and the tensile loss elastic modulus E″ can be controlled by adjusting the molecular weight of a polyol, the molecular weight of a polyisocyanate, a ratio (NCO/OH), and the like.

For measuring the shear loss elastic modulus G″, a sheet having a thickness of 2 mm is obtained by press molding from a resin composition that is the same as the resin composition of the cover 8. A test piece having a width of 10 mm and an inter-clamp distance of 10 mm is punched out from the sheet. The shear loss elastic modulus G″ is measured for the test piece. The measurement conditions are as follows.

Apparatus: “Rheometer ARES”, manufactured by TA instruments

Measurement mode: twisting (shearing)

Measurement temperature: 0° C.

Vibration frequency: 10 Hz

Measurement distortion: 0.1%

For measuring the tensile loss elastic modulus E″, a sheet having a thickness of 2 mm is obtained by press molding from a resin composition that is the same as the resin composition of the cover 8. A test piece having a width of 4 mm and an inter-clamp distance of 20 mm is punched out from the sheet. The tensile loss elastic modulus E″ is measured for the test piece. The measurement conditions are as follows.

Apparatus: the dynamic viscoelasticity measuring apparatus “Rheogel-E4000”, manufactured by UBM

Measurement mode: pulling

Measurement temperature: 0° C.

Vibration frequency: 10 Hz

Measurement distortion: 0.1%

A time for which a golf ball and a club contact each other is several hundred microseconds. Thus, the frequency of deformation of the golf ball 2 when being hit is several thousand Hz. On average, the golf ball 2 is hit at substantially normal temperature (25° C.). On the basis of a general time conversion rule of polyurethane, a deformation having a frequency of several thousand Hz in the environment having a temperature of 25° C. corresponds to a deformation having a frequency of 10 Hz in the environment having a temperature of 0° C. Thus, in the present invention, the shear loss elastic modulus G″ and the tensile loss elastic modulus E″ are measured under the conditions of a vibration frequency of 10 Hz and a temperature of 0° C.

A polyurethane and another resin may be used in combination for the cover 8. In this case, in light of spin performance and feel at impact, the polyurethane is included as the principal component of the base polymer. The proportion of the polyurethane to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 70% by weight, and particularly preferably equal to or greater than 85% by weight.

For the cover 8, thermoplastic polyurethanes and thermosetting polyurethanes can be used. In light of productivity, thermoplastic polyurethanes are preferred. A thermoplastic polyurethane includes a polyurethane component as a hard segment, and a polyester component or a polyether component as a soft segment.

The polyurethane includes a polyol component. As the polyol, a polymeric polyol is preferred. Specific examples of polymeric polyols include polyether polyols such as polyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG), and polytetramethylene ether glycol (PTMG); condensed polyester polyols such as polyethylene adipate (PEA), polybutylene adipate (PBA), and polyhexamethylene adipate (PHMA); lactone polyester polyols such as poly-ε-caprolactone (PCL); polycarbonate polyols such as polyhexamethylene carbonate; and acrylic polyols. Two or more polyols may be used in combination.

Particularly, polytetramethylene ether glycol is preferred. The spin rate obtained when the golf ball 2 is hit with a short iron has a high correlation with the content of polytetramethylene ether glycol. Meanwhile, the spin rate obtained when the golf ball 2 is hit with a driver has a low correlation with the content of polytetramethylene ether glycol. The golf ball 2 including a polyurethane that includes polytetramethylene ether glycol in an appropriate amount has both excellent flight performance when being hit with a driver and excellent controllability when being hit with a short iron.

In light of controllability, the polyol has a number average molecular weight of preferably 200 or greater, more preferably 400 or greater, and particularly preferably 650 or greater. In light of suppression of spin, the molecular weight is preferably equal to or less than 1500, more preferably equal to or less than 1200, and particularly preferably equal to or less than 850.

The number average molecular weight is measured by gel permeation chromatography. The measurement conditions areas follows.

Apparatus: HLC-8120GPC (manufactured by Tosoh Corporation)

Eluant: tetrahydrofuran

Concentration: 0.2% by weight

Temperature: 40° C.

Column: TSK gel Super HM-M (manufactured by Tosoh Corporation)

Sample volume: 5 microliters

Flow rate: 0.5 milliliter/min

Reference material: polystyrene (“PStQuick Kit-H” manufactured by Tosoh Corporation)

The polymeric polyol component has a hydroxyl value of preferably 94 mg KOH/g or greater and particularly preferably 112 mg KOH/g or greater. The hydroxyl value is preferably equal to or less than 561 mg KOH/g and particularly preferably equal to or less than 173 mg KOH/g.

Examples of an isocyanate component in the polyurethane include aromatic polyisocyanates such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, a mixture (TDI) of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate, 4,4′-diphenylmethane diisocyanate (MDI), 1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate (TODI), xylylene diisocyanate (XDI), tetramethylxylene diisocyanate (TMXDI), and paraphenylene diisocyanate (PPDI); alicyclic polyisocyanates such as 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI), hydrogenated xylylene diisocyanate (H₆XDI), and isophorone diisocyanate (IPDI); and aliphatic polyisocyanates such as hexamethylene diisocyanate (HDI). Two or more polyisocyanates may be used in combination. In light of weather resistance, TMXDI, XDI, HDI, H₆XDI, IPDI, and H₁₂MDI are preferred, and H₁₂MDI is particularly preferred.

The polyurethane may include a chain extender as its component. Examples of chain extenders include low-molecular-weight polyols and low-molecular-weight polyamines.

Examples of low-molecular-weight polyols include diols, triols, tetraols, and hexaols. Specific examples of diols include ethylene glycol, diethylene glycol, propanediol, dipropylene glycol, butanediol, neopentyl glycol, pentanediol, hexanediol, heptanediol, and octanediol. Specific examples of triols include glycerin, trimethylolpropane, and hexanetriol. Specific examples of tetraols include pentaerythritol and sorbitol. 1,4-butanediol is preferred.

Examples of low-molecular-weight polyamines include aliphatic polyamines, monocyclic aromatic polyamines, and polycyclic aromatic polyamines. Specific examples of aliphatic polyamines include ethylenediamine, propylenediamine, butylenediamine, and hexamethylenediamine. Specific examples of monocyclic aromatic polyamines include phenylenediamine, toluene diamine, dimethyl toluene diamine, dimethylthio toluene diamine, and xylylenediamine.

The chain extender has a number average molecular weight of preferably 30 or greater, more preferably 40 or greater, and particularly 45 or greater. The molecular weight is preferably equal to or less than 400, more preferably equal to or less than 350, and particularly preferably equal to or less than 200. Low-molecular-weight polyols and low-molecular-weight polyamines that are used as chain extenders are low-molecular-weight compounds that almost do not have a molecular weight distribution. Thus, the low-molecular-weight polyols and the low-molecular-weight polyamines can be distinguished from the polymeric polyol.

The cover 8 may be formed from a composition including a thermoplastic polyurethane and an isocyanate compound. During or after forming the cover 8, the polyurethane is crosslinked with the isocyanate compound.

According to need, a coloring agent such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the cover 8 in an adequate amount.

The cover 8 has a JIS-C hardness Hc of 65 or less. Use of the flexible cover 8 can achieve excellent controllability upon a shot with a short iron. In light of controllability, the hardness Hc is more preferably equal to or less than 60, even more preferably equal to or less than 55, and particularly preferably equal to or less than 50. If the hardness Hc is excessively low, the flight performance upon a shot with a driver is insufficient. In this respect, the hardness Hc is preferably equal to or greater than 20, more preferably equal to or greater than 25, and particularly preferably equal to or greater than 35. For measuring the hardness Hc, a slab formed from a resin composition that is the same as the resin composition of the cover 8 is used. The measurement method is the same as the measurement method for the hardness Hm of the mid layer 6.

The hardness Hc of the cover 8 is less than the central hardness Ho of the core 4. The golf ball 2 has excellent controllability upon a shot with a short iron. In light of controllability, the difference (Ho−Hc) is preferably equal to or greater than 15, more preferably equal to or greater than 17, and particularly preferably equal to or greater than 20. The difference (Ho−Hc) is preferably equal to or less than 40, more preferably equal to or less than 35, and particularly preferably equal to or less than 30.

In light of flight performance upon a shot with a driver, the cover 8 has a thickness of preferably 0.8 mm or less, more preferably 0.6 mm or less, even more preferably 0.5 mm or less, and particularly preferably 0.4 mm or less. In light of controllability upon a shot with a short iron, the thickness is preferably equal to or greater than 0.10 mm and particularly preferably equal to or greater than 0.15 mm.

For forming the cover 8, known methods such as injection molding, compression molding, and the like can be used. When forming the cover 8, the dimples 14 are formed by pimples formed on the cavity face of a mold.

In light of feel at impact, the golf ball 2 has an amount of compressive deformation of preferably 2.3 mm or greater, more preferably 2.4 mm or greater, and particularly preferably 2.5 mm or greater. In light of resilience performance, the amount of compressive deformation is preferably equal to or less than 3.5 mm, more preferably equal to or less than 3.2 mm, and particularly preferably equal to or less than 3.0 mm.

At measurement of the amount of compressive deformation, first, the golf ball 2 is placed on a hard plate made of metal. Next, a cylinder made of metal gradually descends toward the golf ball 2. The golf ball 2, squeezed between the bottom face of the cylinder and the hard plate, becomes deformed. A migration distance of the cylinder, starting from the state in which an initial load of 98 N is applied to the golf ball 2 up to the state in which a final load of 1274 N is applied thereto, is measured.

If the cover 8 is laminated directly on the mid layer 6, the cover 8 does not firmly adhere to the mid layer 6 due to the difference between the material of the cover 8 and the material of the mid layer 6. The golf ball 2 preferably includes an adhesive layer between the mid layer 6 and the cover 8. The adhesive layer firmly adheres to the mid layer 6 and also to the cover 8. The adhesive layer suppresses separation of the cover 8 from the mid layer 6. As described above, the cover 8 of the golf ball 2 is thin. When the golf ball 2 is hit by the edge of a clubface, a wrinkle is likely to occur. The adhesive layer suppresses occurrence of a wrinkle. The golf ball 2 is unlikely to break even by being repeatedly hit. In the golf ball 2, loss of energy transfer is small when the golf ball 2 is hit with a golf club. Thus, the golf ball 2 has excellent resilience performance.

The adhesive layer is formed by applying an adhesive to the surface of the mid layer 6 and drying the adhesive. The base polymer of the adhesive is a two-component curing type epoxy resin. A preferable two-component curing type epoxy resin is obtained by curing a bisphenol A type epoxy resin with a curing agent including a polyamine compound. The bisphenol A type epoxy resin is used for the two-component curing type epoxy resin, and thus the two-component curing type epoxy resin has excellent flexibility, chemical resistance, heat resistance, and toughness.

The adhesive is obtained by mixing a base material including a bisphenol A type epoxy resin and a solvent with a curing agent including a polyamine compound and a solvent. Examples of the solvents in the base material and the curing agent include organic solvents such as xylene and toluene and water.

Specific examples of the polyamine compound include polyamide amines and modified products thereof. A polyamide amine has a plurality of amino groups and one or more amide groups. The amino groups can react with epoxy groups. A polyamide amine can be obtained by a condensation reaction of a polymerized fatty acid and a polyamine. A typical polymerized fatty acid is obtained by heating and combining natural fatty acids including a large amount of unsaturated fatty acids, such as linoleic acid, linolenic acid, and the like, in the presence of a catalyst. Specific examples of unsaturated fatty acids include tall oil, soybean oil, linseed oil, and fish oil. A hydrogenated polymerized fatty acid having a dimer content of 90% by weight or greater and a trimer content of 10% by weight or less is preferred. Examples of preferable polyamines include polyethylene diamines, polyoxyalkylene diamines, and derivatives thereof.

The adhesive has a gel fraction of 40% or greater. In the adhesive layer formed from the adhesive having a gel fraction of 40% or greater, a volatile component is unlikely to remain and thus there are few bubbles included. The adhesive layer firmly adheres to the mid layer 6 and also to the cover 8. In this respect, the gel fraction is more preferably equal to or greater than 45% and particularly preferably equal to or greater than 50%.

The adhesive has a gel fraction of 80% or less. The adhesive having a gel fraction of 80% or less sufficiently reacts with the base polymer of the mid layer 6 and also with the base polymer of the cover 8. The adhesive layer firmly adheres to the mid layer 6 and also to the cover 8. In this respect, the gel fraction is more preferably equal to or less than 76% and particularly preferably equal to or less than 70%.

The adhesive layer formed from the adhesive of which the gel fraction is equal to or greater than 40% but equal to or less than 80% exerts a remarkable effect in the golf ball 2 including the thin cover 8. The adhesive layer formed from the adhesive of which the gel fraction is equal to or greater than 40% but equal to or less than 80% exerts a remarkable effect in the golf ball 2 including the flexible cover 8.

At measurement of the gel fraction, immediately after the base material and the curing agent are mixed, the adhesive is applied to a PB-137T zinc phosphate treated steel plate. The size of the steel plate is “150 mm×70 mm”. The thickness of the steel plate is 0.8 mm. The steel plate is kept in the environment of 40° C. for 24 hours to form a coating film formed from the adhesive. A test piece is obtained from the steel plate and the coating film. The weight of the test piece is measured, and a weight M1 of the coating film is calculated by subtracting the weight of the steel plate from the measurement value. The test piece is immersed in acetone and allowed to stand for 24 hours. The test piece is kept in the environment of 105° C. for one hour. The test piece is cooled to 23° C. The weight of the test piece is measured, and a weight M2 of the coating film is calculated by subtracting the weight of the steel plate from the measurement value. A gel fraction G is calculated by the following mathematical formula. G=(M2/M1)·100

The ratio of the epoxy equivalent of the bisphenol A type epoxy resin to the amine active hydrogen equivalent of the curing agent in the adhesive is preferably equal to or greater than 2.0/1.0 but equal to or less than 13.0/1.0. In the adhesive in which the ratio is equal to or greater than 2.0/1.0, the gel fraction is not too low. Therefore, the adhesive layer firmly adheres to the mid layer 6 and the cover 8. In this respect, the ratio is more preferably equal to or greater than 2.6/1.0 and particularly preferably equal to or greater than 4.0/1.0. In the adhesive in which the ratio is equal to or less than 13.0/1.0, the gel fraction is not too high. Therefore, the adhesive layer firmly adheres to the mid layer 6 and the cover 8. In this respect, the ratio is more preferably equal to or less than 12.2/1.0 and particularly preferably equal to or less than 10.0/1.0.

The amine active hydrogen equivalent of the curing agent is preferably equal to or greater than 100 g/eq but equal to or less than 800 g/eq. In the adhesive in which the equivalent is equal to or greater than 100 g/eq, the gel fraction is not too high. Therefore, the adhesive layer firmly adheres to the mid layer 6 and the cover 8. In this respect, the equivalent is more preferably equal to or greater than 200 g/eq and particularly preferably equal to or greater than 300 g/eq. In the adhesive in which the equivalent is equal to or less than 800 g/eq, the gel fraction is not too low. Therefore, the adhesive layer firmly adheres to the mid layer 6 and the cover 8. In this respect, the equivalent is more preferably equal to or less than 600 g/eq and particularly preferably equal to or less than 500 g/eq.

The adhesive includes water as a volatile component. The term “volatile component” means both water and an organic solvent. The proportion Pw of water to the entire volatile component is preferably equal to or greater than 90% by weight. In the adhesive in which the proportion Pw is equal to or greater than 90% by weight, the gel fraction is easily controlled. In this respect, the proportion Pw is more preferably equal to or greater than 95% by weight and particularly preferably equal to or greater than 99% by weight. The proportion Pw may be 100% by weight. In light of the environment, the proportion Po of the organic solvent to the entire volatile component is preferably equal to or less than 10% by weight, more preferably equal to or less than 5% by weight, and particularly preferably equal to or less than 1% by weight.

The adhesive layer may include additives such as a coloring agent (typically, titanium dioxide), an antioxidant, a light stabilizer, a fluorescent brightener, an ultraviolet absorber, an anti-blocking agent, and the like. The additives may be added to the base material or the curing agent.

As described above, the adhesive layer is obtained by applying the adhesive to the surface of the mid layer 6. The application can be conducted by a spray gun method, an electrostatic coating method, or a dipping method. In light of workability, the application by the spray gun method is preferred. After the application, the solvent is volatilized to permit a reaction of the bisphenol A type epoxy resin with the polyamine compound, thereby forming the adhesive layer.

In light of durability of the golf ball 2, the adhesive layer has a thickness of preferably 0.001 mm or greater and particularly preferably 0.002 mm or greater. The thickness is preferably equal to or less than 0.1 mm. The thickness is measured by observing a cross-section of the golf ball 2 with a microscope. When the mid layer 6 has concavities and convexities on its surface from surface roughening, the thickness is measured at a convex part. The measurement is conducted so as to avoid the positions of the dimples 14.

The adhesive strength between the mid layer 6 (first layer) and the cover 8 (second layer) is preferably equal to or greater than 20 N. The golf ball 2 in which the adhesive strength is equal to or greater than 20 N has excellent durability. In this respect, the adhesive strength is more preferably equal to or greater than 22.0 N and particularly preferably equal to or greater than 22.3 N.

At measurement of the adhesive strength, a test piece including the first layer, the adhesive layer, and the second layer is cut out from the golf ball 2. The size of the test piece is “10 mm×50 mm”. At an end of the test piece, the second layer is peeled from the first layer. The first layer is fixed to a first chuck, and the second layer is fixed to a second chuck. The second chuck is moved relative to the first chuck to peel the first layer from the second layer. The force applied at this peeling is measured. For the measurement, “autograph AG-IS” manufactured by SHIMADZU CORPORATION is used. The tensile rate is 50 mm/min.

Also for a golf ball that includes a center, an envelope layer formed from a resin composition and covering the center, a mid layer formed from a resin composition and covering the envelope layer, and a cover formed from a resin composition and covering the mid layer, an adhesive layer is effective. In the golf ball, an adhesive layer may be present between the envelope layer and the mid layer, and an adhesive layer may be present between the mid layer and the cover. When the adhesive layer is present between the envelope layer and the mid layer, the envelope layer is a first layer, and the mid layer is a second layer. When the adhesive layer is present between the mid layer and the cover, the mid layer is a first layer, and the cover is a second layer.

EXAMPLES Example 1

A rubber composition (1) was obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-730”, manufactured by JSR Corporation), 23 parts by weight of zinc diacrylate, 5 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.5 parts by weight of diphenyl disulfide, and 0.7 parts by weight of dicumyl peroxide. The rubber composition (1) was placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 170° C. for 15 minutes to obtain a center with a diameter of 15 mm.

A rubber composition (2) was obtained by kneading 100 parts by weight of a high-cis polybutadiene (the aforementioned “BR-730”), 33 parts by weight of zinc diacrylate, 5 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.3 parts by weight of bis(pentabromophenyl)disulfide, and 0.9 parts by weight of dicumyl peroxide. Half shells were formed from the rubber composition (2). The center was covered with two half shells. The center and the half shells were placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 150° C. for 20 minutes to obtain a core with a diameter of 39.7 mm. An envelope layer was formed from the rubber composition (2). The amount of barium sulfate was adjusted such that the specific gravity of the envelope layer agrees with the specific gravity of the center and the weight of a golf ball is 45.4 g.

A resin composition (a) was obtained by kneading 50 parts by weight of an ionomer resin (the aforementioned “Surlyn 8945”) and 50 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”) with a twin-screw kneading extruder. The core was placed into a mold including upper and lower mold halves each having a hemispherical cavity. The core was covered with the resin composition (a) by injection molding to form a mid layer with a thickness of 1.0 mm.

An adhesive including a base material and a curing agent was prepared. The base material is a water-based epoxy composition manufactured by SHINTO PAINT CO., LTD. The base material includes 36 parts by weight of a bisphenol A type epoxy resin and 64 parts by weight of water. The epoxy equivalent of the base material is 1405 g/eq. The curing agent is a water-based amine composition manufactured by SHINTO PAINT CO., LTD. The curing agent includes 44 parts by weight of a modified polyamide amine, 50 parts by weight of water, 1 parts by weight of propylene glycol, and 5 parts by weight of titanium dioxide. The active hydrogen equivalent of the curing agent is 348 g/eq. This adhesive was applied to the surface of the mid layer with a spray gun, and kept at 23° C. for 12 hours to obtain an adhesive layer with a thickness of 0.003 mm.

A resin composition (b) was obtained by kneading 100 parts by weight of a polyurethane (trade name “Elastollan XNY85A”, manufactured by BASF Japan Ltd.) and 4 parts by weight of titanium dioxide with a twin-screw kneading extruder. Half shells were obtained from the resin composition (b) by compression molding. The sphere consisting of the core, the mid layer, and the reinforcing layer was covered with two of these half shells. The sphere and the half shells were placed into a final mold that includes upper and lower mold halves each having a hemispherical cavity and that has a large number of pimples on its cavity face. A cover was obtained by compression molding. The cover had a thickness of 0.5 mm. Dimples having a shape that was the inverted shape of the pimples were formed on the cover. A clear paint including a two-component curing type polyurethane as a base material was applied to this cover to obtain a golf ball of Example 1 with a diameter of 42.7 mm.

Examples 2 to 7 and Comparative Examples 1 to 4

Golf balls of Examples 2 to 7 and Comparative Examples 1 to 4 were obtained in the same manner as Example 1, except the specifications of the center, the envelope layer, the mid layer, and the cover were as shown in Tables 5 to 7 below. The rubber composition of the core is shown in detail in Table 1 below. The resin compositions of the mid layer and the cover are shown in detail in Table 2 below. A hardness distribution of the core is shown in Tables 3 and 4 below. The golf ball according to Comparative Example 1 does not have an envelope layer.

[Shot with Driver (W#1)]

A driver with a titanium head (trade name “SRIXON Z-TX”, manufactured by SRI Sports Limited, shaft hardness: X, loft angle: 8.5°) was attached to a swing machine manufactured by Golf Laboratories, Inc. A golf ball was hit under the condition of a head speed of 50 m/sec. The ball speed and the spin rate immediately after the hit and the distance from the launch point to the stop point were measured. The average value of data obtained by 12 measurements is shown in Tables 5 to 7 below.

[Shot with Middle Iron (I#5)]

A 5-iron (trade name “SRIXON Z-TX (steel shaft), manufactured by SRI Sports Limited) was attached to the swing machine. A golf ball was hit under the condition of a head speed of 41 m/sec. The ball speed and the spin rate immediately after the hit and the distance from the launch point to the stop point were measured. The average value of data obtained by 12 measurements is shown in Tables 5 to 7 below.

[Shot with Short Iron (SW)]

A sand wedge (trade name “CG15 Chrome Wedge”, manufactured by SRI Sports Limited, loft angle: 58°) was attached to the swing machine. A golf ball was hit under the condition of a head speed of 21 m/sec, and the spin rate was measured immediately after the hit. The average value of data obtained by 12 measurements is shown in Tables 5 to 7 below. In addition, water was applied to a clubface and a golf ball, and the golf ball was hit. The spin rate was measured immediately after the hit. The average value of data obtained by 12 measurements is shown in Tables 5 to 7 below.

[Feel at Impact]

Ten golf players hit golf balls with sand wedges, and were asked about feel at impact. The evaluation was categorized as follows on the basis of the number of golf players who answered, “the feel at impact was excellent”.

A: 8 or more

B: 6 to 7

C: 4 to 5

D: 3 or less

The results are shown in Tables 5 to 7 below.

TABLE 1 Composition of Core (1) (2) (3) (4) (5) (6) BR730 100 100 100 100 100 100 Zinc diacrylate 23 33 — 30 35 30 Methacrylic acid — — 28.0 — — — Magnesium oxide — — 34.8 — — — Zinc oxide 5 5 — 5 5 5 Barium sulfate * * — * * * Diphenyl disulfide 0.5 — — — — — Bis(pentabromophenyl) — 0.3 — 0.3 0.3 0.3 disulfide Dicumyl peroxide 0.7 0.9 1.3 1.05 0.9 0.9 * Appropriate amount

TABLE 2 Compositions of Mid Layer and Cover (a) (b) (c) (d) (e) (f) (g) Surlyn 8945 50 — — — — — — Himilan AM7329 50 — — — — — — Polyurethane *1 — 100 — — — — — Polyurethane *2 — — 100 — — — — Polyurethane *3 — — — 100 — — — Polyurethane *4 — — — — 100 — — Polyurethane *5 — — — — — 100 — Polyurethane *6 — — — — — — 100 Titanium dioxide — 4 4 4 4 4 4 Molecular weight — 1800 1800 1800 1500 1500 1000 of PTMG E″ (×10⁷ Pa) — 3.24 4.61 7.57 1.80 3.51 6.26 G″ (×10⁷ Pa) — 2.77 3.23 4.01 1.02 1.42 3.00 E″/G″ — 1.17 1.43 1.89 1.76 2.47 2.09 Hardness (JIS-C) 94 47 56 67 45 47 44 Hardness (Shore D) 64 32 38 47 30 32 29

The details of the polyurethanes in Table 2 are as follows. Polyurethane *1

Trade name “Elastollan XNY85A”, manufactured by BASF Japan Ltd.

Polyol component: polytetramethylene ether glycol

Number average molecular weight of polyol component: 1800 Polyurethane *2

Trade name “Elastollan XNY90A”, manufactured by BASF Japan Ltd.

Polyol component: polytetramethylene ether glycol

Number average molecular weight of polyol component: 1800 Polyurethane *3

Trade name “Elastollan XNY97A”, manufactured by BASF Japan Ltd.

Polyol component: polytetramethylene ether glycol

Number average molecular weight of polyol component: 1800 Polyurethane *4

Thermoplastic polyurethane elastomer

Polyol component: polytetramethylene ether glycol

Number average molecular weight of polyol component: 1500 Polyurethane *5

Thermoplastic polyurethane elastomer

Polyol component: polytetramethylene ether glycol

Number average molecular weight of polyol component: 1500 Polyurethane *6

Thermoplastic polyurethane elastomer

Polyol component: polytetramethylene ether glycol

Number average molecular weight of polyol component: 1000

TABLE 3 Hardness Distribution of Core (JIS-C) Distance from central point Example (mm) 1 2 3 4 5 6 7 8 0 65.0 65.0 65.0 65.0 65.0 65.0 65.0 65.0 5.0 67.0 67.0 67.0 67.0 67.0 67.0 67.0 67.0 6.5 68.0 — 68.0 68.0 68.0 68.0 68.0 68.0 7.5 68.5 — 68.5 68.5 68.5 68.5 68.5 68.5 7.5 70.5 — 70.5 70.5 70.5 70.5 70.5 67.5 8.0 — 68.7 — — — — — — 8.5 72.0 — 72.0 72.0 72.0 72.0 72.0 69.0 9.0 — 69.5 — — — — — — 9.0 — 72.0 — — — — — — 10.0 74.0 73.5 74.0 74.0 74.0 74.0 74.0 71.0 15.0 81.5 81.5 81.5 81.5 81.5 81.5 81.5 78.5 19.9 86.0 — — — — — — 83.0 20.0 — 86.0 86.0 86.0 86.0 86.0 86.0 —

TABLE 4 Hardness Distribution of Core (JIS-C) Distance from central point Comparative Example (mm) 1 2 3 4 0 66.0 65.0 65.0 65.0 5.0 72.0 66.5 67.0 67.0 6.5 — — 68.0 68.0 7.5 — — 68.5 68.5 7.5 — — 67.5 70.5 8.5 — — 69.0 72.0 10.0 72.0 69.5 71.0 74.0 12.5 — 71.5 — — 12.5 — 75.0 — — 15.0 78.0 79.0 78.5 81.5 19.5 — 87.0 83.0 — 20.0 84.0 — — 86.0

TABLE 5 Results of Evaluation Example 1 Example 2 Example 3 Example 4 Center Composition (1) (1) (1) (1) Crosslinking 170 170 170 170 temperature (° C.) Crosslinking time (min) 15 15 15 15 Diameter (mm) 15 18 15 15 Envelope Composition (2) (2) (2) (2) layer Crosslinking 150 150 150 150 temperature (° C.) Crosslinking time (min) 20 20 20 20 Core Diameter (mm) 39.7 40.1 40.3 40.1 Volume ratio (%) 80.4 82.8 82.8 82.8 Central hardness Ho 65 65 65 65 (JIS-C) Surface hardness He 86 86 86 86 (JIS-C) He − Ho 21 21 21 21 Hardness distribution FIG. 2 FIG. 3 FIG. 4 FIG. 4 H2-H1 (minimum value) 0.8 0.8 0.8 0.8 H2-H1 (maximum value) 4.8 4.8 4.0 4.0 Mid Composition (a) (a) (a) (a) layer Hardness Hm (JIS-C) 94 94 94 94 Thickness (mm) 1.0 1.0 0.9 1.0 Cover Composition (b) (c) (b) (e) Hardness Hc (JIS-C) 47 56 47 45 Thickness (mm) 0.5 0.3 0.3 0.3 Molecular weight of PTMG 1800 1800 1800 1500 E″ (×10⁷ Pa) 3.24 46.1 3.24 1.80 G″ (×10⁷ Pa) 2.77 3.23 2.77 1.02 E″/G″ 1.17 1.43 1.17 1.76 Ball Deformation (mm) 2.45 2.45 2.45 2.45 W#1 Ball speed (m/s) 73.9 74.1 74.0 74.0 Spin (rpm) 2550 2470 2480 2500 Flight distance (m) 260.0 261.0 261.0 260.5 I#5 Ball speed (m/s) 58.0 58.3 58.2 58.2 Spin (rpm) 4890 4800 4820 4850 Flight distance (m) 182.5 183.2 183.1 183.0 SW Dry spin (rpm) 6600 6500 6530 6550 Wet spin (rpm) 4300 4200 4280 4400 Feel at impact A B A A

TABLE 6 Results of Evaluation Example 5 Example 6 Example 7 Example 8 Center Composition (1) (1) (3) (1) Crosslinking 170 170 170 170 temperature (° C.) Crosslinking time (min) 15 15 20 15 Diameter (mm) 15 15 15 15 Envelope Composition (2) (2) (2) (6) layer Crosslinking 150 150 150 150 temperature (° C.) Crosslinking time (min) 20 20 20 20 Core Diameter (mm) 40.1 40.1 40.1 39.7 Volume ratio (%) 82.8 82.8 82.8 80.4 Central hardness Ho 65 65 65 65 (JIS-C) Surface hardness He 86 86 86 83 (JIS-C) He − Ho 21 21 21 18 Hardness distribution FIG. 4 FIG. 4 FIG. 4 FIG. 5 H2-H1 (minimum value) 0.8 0.8 0.8 0.8 H2-H1 (maximum value) 4.0 4.0 4.0 3.0 Mid Composition (a) (a) (a) (a) layer Hardness Hm (JIS-C) 94 94 94 94 Thickness (mm) 1.0 1.0 1.0 1.2 Cover Composition (f) (g) (e) (e) Hardness Hc (JIS-C) 42 42 45 45 Thickness (mm) 0.5 0.5 0.3 0.3 Molecular weight of PTMG 1500 1000 1500 1500 E″ (×10⁷ Pa) 3.51 6.26 1.80 1.80 G″ (×10⁷ Pa) 1.42 3.00 1.02 1.02 E″/G″ 2.47 2.09 1.76 1.76 Ball Deformation (mm) 2.45 2.45 2.45 2.50 W#1 Ball speed (m/s) 74.0 73.9 74.0 73.8 Spin (rpm) 2530 2540 2500 2460 Flight distance (m) 260.3 260.2 260.5 260.0 I#5 Ball speed (m/s) 58.1 58.1 58.2 57.9 Spin (rpm) 4870 4890 4850 4790 Flight distance (m) 182.7 182.6 183.0 182.1 SW Dry spin (rpm) 6700 6680 6550 6490 Wet spin (rpm) 4600 4570 4400 4380 Feel at impact A A A A

TABLE 7 Results of Evaluation Compa. Compa. Compa. Compa. Example 1 Example 2 Example 3 Example 4 Center Composition (4) (1) (1) (1) Crosslinking 170 170 170 170 temperature (° C.) Crosslinking time (min) 20 15 15 15 Diameter (mm) 39.7 25 15 15 Envelope Composition — (5) (6) (2) layer Crosslinking — 150 150 150 temperature (° C.) Crosslinking time (min) — 20 20 20 Core Diameter (mm) 39.7 39.1 38.5 40.1 Volume ratio (%) 80.4 76.8 73.3 82.8 Central hardness Ho 66 64 63 66 (JIS-C) Surface hardness He 84 87 83 86 (JIS-C) He − Ho 18 23 20 20 Hardness distribution FIG. 6 FIG. 7 FIG. 8 FIG. 4 H2-H1 (minimum value) 0.0 0.6 0.8 0.8 H2-H1 (maximum value) 2.4 5.9 3.0 4.0 Mid Composition (a) (a) (a) (a) layer Hardness Hm (JIS-C) 94 94 94 94 Thickness (mm) 1.0 1.0 1.6 1.0 Cover Composition (b) (b) (b) (d) Hardness Hc (JIS-C) 47 47 47 67 Thickness (mm) 0.5 0.8 0.5 0.3 Molecular weight of PTMG 1800 1800 1800 1800 E″ (×10⁷ Pa) 3.24 3.24 3.24 7.57 G″ (×10⁷ Pa) 2.77 2.77 2.77 4.01 E″/G″ 1.17 1.17 1.17 1.89 Ball Deformation (mm) 2.45 2.45 2.40 2.45 W#1 Ball speed (m/s) 73.7 73.5 73.8 74.2 Spin (rpm) 2600 2650 2680 2440 Flight distance (m) 259.1 258.6 258.9 259.9 I#5 Ball speed (m/s) 57.7 57.5 57.8 58.4 Spin (rpm) 4950 5000 4980 4810 Flight distance (m) 181.9 181.3 181.7 182.9 SW Dry spin (rpm) 6630 6700 6710 6300 Wet pin (rpm) 4290 4350 4370 4000 Feel at impact A A B C

As shown in Tables 5 to 7, the golf balls according to Examples are excellent in various performance characteristics. From the results of evaluation, advantages of the present invention are clear.

The golf ball according to the present invention can be used for playing golf on golf courses and practicing at driving ranges. The above descriptions are merely for illustrative examples, and various modifications can be made without departing from the principles of the present invention. 

What is claimed is:
 1. A golf ball comprising a core, a mid layer positioned outside the core, and a cover positioned outside the mid layer, wherein the core comprises a center and an envelope layer positioned outside the center, the hardness of the center hardness gradually increases from its central point to its surface, the hardness of the envelope layer gradually increases from its innermost portion to its surface, a ratio of a volume of the core to a volume of a phantom sphere of the golf ball is equal to or greater than 76%, a difference (He−Ho) between a JIS-C hardness He at a surface of the core and a JIS-C hardness Ho at a central point of the core is equal to or greater than 15 but equal to or less than 30, a JIS-C hardness He of the cover is less than the hardness Ho, at all points P included in a zone that extends over a distance range from equal to or greater than 1 mm to equal to or less than 15 mm from the central point of the core, the following mathematical formula is satisfied, −5<H2−H1<5 and in the mathematical formula, H1 indicates a JIS-C hardness at a point P1 that is located radially inward of each point P at a distance of 1 mm from the point P, and H2 indicates a JIS-C hardness at a point P2 that is located radially outward of each point P at a distance of 1 mm from the point P.
 2. The golf ball according to claim 1, wherein the JIS-C hardness He of the cover is equal to or less than
 65. 3. The golf ball according to claim 1, wherein a thickness of the cover is equal to or less than 0.8 mm.
 4. The golf ball according to claim 1, wherein a JIS-C hardness Hm of the mid layer is equal to or greater than
 90. 5. The golf ball according to claim 1, wherein a thickness of the mid layer is equal to or less than 1.5 mm.
 6. The golf ball according to claim 1, wherein the cover is formed from a resin composition, and a principal component of a base material of the resin composition is a thermoplastic polyurethane.
 7. The golf ball according to claim 6, wherein a shear loss elastic modulus G″ of the resin composition, which is measured under conditions of a vibration frequency of 10 Hz and a temperature of 0° C., is equal to or less than 1.95×10⁷ Pa, and a ratio (E″/G″) of a tensile loss elastic modulus E″ of the resin composition, which is measured under conditions of a vibration frequency of 10 Hz and a temperature of 0° C., to the shear loss elastic modulus G″ is equal to or greater than 1.76.
 8. The golf ball according to claim 7, wherein the tensile loss elastic modulus E″ is equal to or greater 2.00×10⁷ Pa but equal to or less than 1.00×10⁸ Pa.
 9. The golf ball according to claim 6, wherein a polyol component of the thermoplastic polyurethane is polytetramethylene ether glycol having a number average molecular weight of 1500 or less.
 10. The golf ball according to claim 6, wherein an isocyanate component of the thermoplastic polyurethane is 4,4′-dicyclohexylmethane diisocyanate.
 11. The golf ball according to claim 1, wherein a diameter of the center is equal to or greater than 10 mm but equal to or less than 20 mm, a thickness of the envelope layer is equal to or greater than 8 mm but equal to or less than 18 mm, a thickness of the mid layer is equal to or less than 1.2 mm, and a thickness of the cover is equal to or less than 0.5 mm.
 12. The golf ball according to claim 1, wherein a difference (Ho−Hc) between the hardness Ho and the hardness He is equal to or greater than 15 but equal to or less than
 40. 13. The golf ball according to claim 1, wherein the center is formed by a rubber composition being crosslinked, and the rubber composition of the center does not include an organic sulfur compound, and the envelope layer is formed by a rubber composition being crosslinked, and the rubber composition of the envelope layer includes an organic sulfur compound.
 14. The golf ball according to claim 1, further comprising an adhesive layer positioned between the mid layer and the cover, wherein the adhesive layer is formed from an adhesive, a base polymer of the adhesive is a two-component curing type epoxy resin obtained by curing a bisphenol A type epoxy resin with a curing agent including a polyamine compound, and a gel fraction of the adhesive is equal to or greater than 40% but equal to or less than 80%.
 15. The golf ball according to claim 14, wherein a ratio of an epoxy equivalent of the bisphenol A type epoxy resin to an amine active hydrogen equivalent of the curing agent in the adhesive is equal to or greater than 2.0/1.0 but equal to or less than 13.0/1.0.
 16. The golf ball according to claim 14, wherein a thickness of the adhesive layer is equal to or greater than 0.001 mm but equal to or less than 0.1 mm.
 17. The golf ball according to claim 1, wherein the hardness Ho is equal to or greater than 40 but equal to or less than
 80. 18. The golf ball according to claim 1, wherein the hardness He is equal to or greater than 70 but equal to or less than
 90. 19. The golf ball according to claim 1, wherein a difference (He-Hi) between the hardness He and a JIS-C hardness Hi at an innermost portion of the envelope layer is equal to or greater than 10 but equal to or less than
 25. 20. The golf ball according to claim 1, wherein at all the points P included in the zone that extends over the distance range from equal to or greater than 1 mm to equal to or less than 15 mm from the central point of the core, the following mathematical formula is satisfied, 0<H2−H1<3. 